US6646788B2 - System and method for wide band Raman amplification - Google Patents

Abstract

A multi-stage Raman amplifier includes a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths, and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage. The second sloped gain profile is approximately complementary slope to the slope of the first sloped gain profile. The combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 09/811,103, filed Mar. 16, 2001, now U.S. Pat. No. 6,532,101, by Mohammed N. Islam, Carl A. DeWilde, and Michael J. Freeman and entitled “SYSTEM AND METHOD FOR WIDE BAND RAMAN AMPLIFICATION”.

STATEMENT OF OTHER APPLICATIONS

This application discloses subject matter that is in some respects similar to that disclosed in copending application Ser. No. 09/811,067, entitled Method and System for Reducing Degradation of Optical Signal to Noise Ratio, filed Mar. 16, 2001 and now U.S. Pat. No. 6,532,101.

This application also discloses subject matter that is in some respects similar to that disclosed in copending application Ser. No. 09/768,367, entitled All Band Amplifier, filed Jan. 22, 2001. application Ser. No. 09/768,367 is a continuation-in-part of U.S. application Ser. No. 09/719,591, filed Dec. 12, 2000, which claims the benefit of copending application serial number PCT/US99/13551, entitled Dispersion Compensating and Amplifying Optical Element, Method for Minimizing Gain Tilt and Apparatus for Minimizing Non-Linear Interaction Between Band Pumps, filed on Jun. 16, 1999, and published on Dec. 23, 1999 as WO 99/66607, which in turn claims the benefit of U.S. application serial No. 60/089,426.

This application and U.S. application Ser. Nos. 09/768,367 and 09/811,067 are currently assigned to Xtera Communications, Inc.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of communication systems, and more particularly to a system and method operable to facilitate wide band optical amplification while maintaining acceptable noise figures.

BACKGROUND OF THE INVENTION

Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.

To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals in parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, broadband optical amplifier systems that permit simultaneous amplification of many WDM channels are a key enabler for utilizing the full fiber bandwidth.

Traditionally, amplification of signals having a broad range of wavelengths has required separating the signals into subsets of wavelengths, and amplifying each subset with a separate amplifier. This approach can be complex and expensive. Using separate amplifiers for each subset requires additional hardware, additional laser pumps for each amplifier, and additional power to launch the additional pumps.

Although a more efficient approach would be to amplify the entire signal using a single amplifier for at least some amplifiers in the system, unfortunately, no acceptable single amplifier approach has been developed. For example, erbium doped-amplifiers are an inherently bad choice for wide band amplification if the ultimate goal is to provide an amplifier that can operate over the entire telecommunications spectrum. For example, for wavelengths shorter than about 1525 nanometers, erbium-atoms in typical glasses will absorb more than they amplify. Even with use of various dopings, such as, aluminum or phosphorus, the absorption peak for the various glasses is still around 1530 nanometers. This leaves a large gap in the short communications band (S-Band) unreachable by erbium doped fiber amplifiers.

Raman amplifiers provide a better solution in terms of broadband amplification potential, but conventional Raman amplifiers have suffered from other shortcomings. For example, Raman amplifiers have traditionally suffered from high noise figures when used in wide band applications. In addition, Raman amplifiers suffer from gain tilt introduced when longer wavelength signals rob energy from shorter wavelength signals. This effect becomes increasingly pronounced as amplifier launch power and system bandwidth increases. Wide band Raman amplifiers operating at high launch powers on a wide range of wavelengths can be particularly vulnerable to this effect.

Masuda, et al. (see e.g., U.S. Pat. No. 6,172,803 B1 and related research papers) have attempted to improve the bandwidth of erbium doped amplifiers by cascading with the erbium doped amplifier a Raman amplifier with an approximately complementary gain profile. Masuda, et al, however, consistently require the presence of an erbium doped amplifier (which relies on different physics for amplification and does not suffer from the same noise problems as Raman amplifiers do) to provide virtually all amplification to signal wavelengths close in spectrum to the pump wavelengths. Indeed, Masuda, et al. concede that the noise figures they report ignore the effect of the Raman portion of their amplifier.

SUMMARY OF THE INVENTION

The present invention recognizes a need for a method and apparatus operable to facilitate wide band Raman amplification while maintaining an approximately flat gain profile and an acceptable noise figure.

In accordance with the present invention, a system and method for providing wide band Raman amplification are provided that substantially reduce or eliminate at least some of the shortcomings associated with prior approaches. In one aspect of the invention, a multi-stage Raman amplifier comprises a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths, and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage. The second sloped gain profile has an approximately complementary slope to the slope of the first sloped gain profile. The combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.

In another aspect of the invention, a method of amplifying an optical signal having multiple wavelengths comprises amplifying a plurality of signal wavelengths at a first Raman amplifier stage having a first sloped gain profile, and amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage after those signal wavelengths have been amplified by the first stage. The second stage has a second sloped gain profile comprising an approximately complimentary gain profile to the first gain profile. The combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.

In still another aspect of the invention, a multi-stage Raman amplifier comprises a plurality of cascaded Raman amplifier stages each having a gain profile, wherein the gain profile of at least some of the Raman stages is sloped. At least two of the sloped gain profiles comprise approximately complimentary gain profiles, wherein the combined effect of the gain profiles of the amplification stages results in an approximately flat overall gain profile over a plurality of signal wavelengths amplified by the amplifier.

In yet another aspect of the invention, a method of amplifying multiple-wavelength optical signals comprises applying a first sloped gain profile to a plurality of signal wavelengths at a first stage of a Raman amplifier, and applying a second sloped gain profile to at least most of the plurality of signal wavelengths at a second stage of the Raman amplifier. The second gain profile comprises an approximately complementary gain profile of the first sloped gain profile. The combined effect of the first and second sloped gain profiles contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.

In another aspect of the invention, a multi-stage Raman amplifier comprises a plurality of cascaded Raman amplifier stages each operable to amplify a plurality of signal wavelengths and each having a gain profile determined at least in part by one or more pump wavelengths applied to the amplifier stage. The plurality of amplifier stages comprise a first Raman stage operable to apply a higher gain level to a signal wavelength closest to a longest pump wavelength than a gain applied to a signal wavelength furthest from the longest pump wavelength.

In still another aspect of the invention, a method of amplifying an optical signal having multiple wavelengths comprises receiving a plurality of signal wavelengths at a plurality of cascaded Raman amplifier stages having at least a first stage and a last stage, where each stage is operable to amplify a plurality of signal wavelengths and each stage has a gain profile determined at least in part by one or more pump wavelengths applied to the amplifier stage. The method further includes applying a highest level of gain supplied by the longest pump wavelength in the last Raman stage of the amplifier.

In yet another aspect of the invention, a multi-stage Raman amplifier comprises a plurality of cascaded Raman amplifier stages, at least some of the Raman stages having sloped gain profiles operable to contribute to a combined gain profile of the amplifier. The combined gain profile of the amplifier is approximately flat across a bandwidth of at least eighty nanometers and comprises a small signal noise figure no greater than eight decibels.

In another aspect of the invention, a method of amplifying an optical signal having multiple wavelengths comprises amplifying a plurality of signal wavelengths at a first Raman amplifier stage having a first sloped gain profile, and amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage having a second sloped gain profile that is different than the first sloped gain profile. The combined gain profile of the amplifier is approximately flat across a bandwidth of at least eighty nanometers and comprises a small signal noise figure no greater than eight decibels.

In another aspect of the invention, an optical pre-amplifier operable to be coupled to an optical communication link carrying optical signals having a plurality of wavelengths comprises a first Raman stage having a gain profile where a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths. The preamplifier further comprises a second Raman stage operable to receive at least most of the signal wavelengths after they have been amplified by the first stage, the second stage having a gain profile where a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths. In this embodiment, the gain profiles of the first and second Raman stages are operable to combine to contribute to an approximately flat combined gain profile over the plurality of signal wavelengths.

In still another aspect of the invention, an optical booster amplifier operable to be coupled to an optical communication link carrying optical signals having a plurality of wavelengths comprises a first Raman stage having a gain profile where a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths. The booster amplifier also comprises a second Raman stage operable to receive at least most of the signal wavelengths after they have been amplified by the first stage, the second stage having a gain profile where a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths. The gain profiles of the first and second Raman stages are operable to combine to contribute to an approximately flat combined gain profile over the plurality of wavelengths.

In yet another aspect of the invention, a Raman amplifier assembly comprises a preamplifier coupled to an optical communication link. The preamplifier includes a first Raman stage having a gain profile wherein a majority of shorter wavelengths are amplified more than a majority of longer wavelengths, and a second Raman stage having a gain profile approximately complementary to the first gain stage. The amplifier assembly also includes a booster amplifier coupled to the optical communication link. The booster amplifier comprises a first Raman stage having a gain profile wherein a majority of longer wavelengths are amplified more than a majority of shorter wavelengths, and a second Raman stage having a gain profile approximately complementary to the first gain stage.

In another aspect of the invention, an optical communication system operable to facilitate communication of multiple signal wavelengths comprises a transmitter bank operable to generate a plurality of signal wavelengths, and a multiplexer operable to combine the plurality of signal wavelengths into a single multiple wavelength signal for transmission over a transmission medium. The system further comprises an amplifier coupled to the transmission medium and operable to amplify the multiple wavelength signal prior to, during, or after the multiple wavelength signal's transmission over the transmission medium, the amplifier comprising a multi-stage Raman amplifier. The amplifier includes a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage. The second sloped gain profile has an approximately complementary slope to the slope of the first sloped gain profile, and the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths. In one embodiment, the system further includes a demultiplexer operable to receive the multiple wavelength signal and to separate the signal wavelengths from the multiple wavelength signal, and a receiver bank operable to receive the plurality of signal wavelengths.

Depending on the specific features implemented, particular embodiments of the present invention may exhibit some, none, or all of the following technical advantages. For example, one aspect of the invention facilitates optical amplification of a wide bandwidth of wavelengths while maintaining an approximately flat gain profile and an acceptable noise figure.

In a particular embodiment, one aspect of the invention reduces the noise figure associated with the amplifier by amplifying in a first Raman stage a majority of shorter wavelengths more than a majority of longer wavelengths. In this way, shorter wavelengths (which are often closest to the pump wavelength) are amplified to overcome any effects that might be caused by phonon-stimulated noise. As a further enhancement, the amplifier could be designed so that the longest pump wavelength is at least ten nanometers below the shortest signal being amplified.

In addition to yielding an acceptable noise figure, this approach can produce an approximately flat gain tilt, for example, by cascading a second Raman amplifier stage having a gain profile that amplifies a majority of longer wavelengths more than a majority of shorter wavelengths. In a particular embodiment, the second gain profile can be approximately complementary to the first gain profile. In some applications, the second gain profile can have an approximately equal (although opposite) slope from the first gain profile.

Another aspect of the invention results in increased efficiency in a multi-stage Raman amplifier. This aspect of the invention involves applying, in at least one Raman stage, a first gain profile that amplifies a majority of longer wavelengths more than a majority of shorter wavelengths; and applying, in a later cascaded Raman stage, a second gain profile that amplifies a majority of shorter wavelengths more than a majority of longer wavelengths. This embodiment facilitates allowing longer pump wavelengths in the first stage to accept energy from shorter pump wavelengths in the later Raman stage. This effect, in turn, facilitates using smaller pump wavelengths and/or fewer pump wavelengths in the first stage than would otherwise be required, thereby increasing the efficiency of the device. In a particular embodiment, the gain profiles of the first and later Raman stages can be approximately complimentary, contributing to an approximately flat overall gain profile for the amplifier. The noise figure can be reduced, for example, by performing a majority of the amplification of wavelengths closest to the pump wavelengths in one of the final amplifier stages, or in the last amplifier stage.

Other aspects of the invention facilitate cascading multiple amplifier stages to realize advantages of low noise and high efficiency in a multiple stage Raman amplifier. Moreover, cascaded stages can provide mid-stage access to the amplifier to facilitate, for example, optical add/drop multiplexing of WDM signals while maintaining an acceptable noise figure and an approximately flat gain profile, both at the mid-stage access point and across the entire amplifier.

Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and for further features and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing an exemplary optical communication system operable to facilitate communication of wide band optical signals constructed according to the teachings of the present invention;

FIG. 2 is a graphical illustration of the phonon-stimulated optical noise figure;

FIG. 3ais a block diagram of an exemplary embodiment of a multiple stage Raman amplifier constructed according to the teachings of the present invention;

FIGS. 3b-3cshow gain profiles associated with various amplification stages and an overall gain profile for the amplifier shown in FIG. 3a, respectively, constructed according to the teachings of the present invention;

FIG. 4ais a block diagram of an exemplary embodiment of a multiple stage Raman amplifier constructed according to the teachings of the present invention;

FIGS. 4b-4cshow gain profiles associated with various amplification stages and an overall gain profile for the amplifier shown in FIG. 4a, respectively, constructed according to the teachings of the present invention;

FIG. 5ais a block diagram of an exemplary embodiment of a three stage Raman amplifier constructed according to the teachings of the present invention;

FIGS. 5b-5cshow gain profiles associated with various amplification stages and an overall gain profile for the amplifier shown in FIG. 5a, respectively, constructed according to the teachings of the present invention;

FIGS. 6ais a block diagram of an exemplary embodiment of a four stage Raman amplifier constructed according to the teachings of the present invention;

FIGS. 6b-6cshow gain profiles associated with various amplification stages and an overall gain profile for the amplifier of FIG. 6a, respectively, constructed according to the teachings of the present invention;

FIG. 7 is a flow chart illustrating one example of a method of amplifying a plurality of wavelengths using a multi-stage Raman amplifier according to the teachings of the present invention;

FIGS. 8a-8bshow simulated gain and noise profiles for one embodiment of a multi-stage hybrid Raman amplifier constructed according to the teachings of the present invention; and

FIGS. 9a-9bshow simulated gain and noise profiles for one embodiment of a multi-stage discrete Raman amplifier constructed according to the teachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram showing an exemplaryoptical communication system10 operable to facilitate communication of wide band optical signals.System10 includes atransmitter bank12 operable to generate a plurality ofwavelength signals16a-16n.Transmitter bank12 may include, for example, a plurality of laser diodes or semiconductor lasers. Eachwavelength signal16a-16ncomprises at least one wavelength of light unique from wavelengths carried byother signals16.

System10 also includes acombiner14 operable to receivemultiple signal wavelengths16a-16nand to combine those signal wavelengths into a singlemultiple wavelength signal16. As one particular example,combiner14 could comprise a wavelength division multiplexer (WDM). The term wavelength division multiplexer as used herein may include conventional wavelength division multiplexers or dense wavelength division multiplexers.

In one particular embodiment,system10 may include abooster amplifier18 operable to receive and amplify wavelengths ofsignal16aprior to communication over atransmission medium20.Transmission medium20 can comprisemultiple spans20a-20nof fiber. As particular examples, fiber spans20 could comprise standard single mode fiber (SMF), dispersion-shifted fiber (DSF), non-zero dispersion-shifted fiber (NZDSF), or other fiber type or combinations of fiber types.

Wherecommunication system10 includes a plurality of fiber spans20a-20n,system10 can include one or moreinline amplifiers22a-22m.Inline amplifiers22 reside between fiber spans20 and operate to amplifysignal16 as it traversesfiber20.

Optical communication system10 can also include apreamplifier24 operable to receivesignal16 from afinal fiber span20nand to amplifysignal16 prior to passing that signal to aseparator26.Separator26 may comprise, for example, a wavelength division demultiplexer (WDM), which can operate on wavelength division multiplexed signals or dense wavelength division multiplexed signals.Separator26 operates to separateindividual wavelength signals16a-16nfrommultiple wavelength signal16.Separator26 communicatesindividual signal wavelength16a-16nto a bank of receivers28.

At least one amplifier insystem10 comprises a wide band multi-stage Raman amplifier operable to receive a wide bandwidth ofwavelength signal16. In a particular embodiment, the amplifier can process over 80 nanometers of bandwidth, and in some cases over 100 nanometers of bandwidth while maintaining an approximately flat gain profile over the bandwidth of amplifiedsignal wavelengths16.

Throughout this document, the term “approximately flat” describes a condition where the maximum signal gain differs from the minimum signal gain by an no more than amount suitable for use in telecommunication systems. The deviation between minimum and maximum signal gains may comprise, for example five decibels prior to application of any gain flattening filters. Particular embodiments of the invention may achieve gain flatness of approximately three decibels prior to application of any gain flattening filters.

Some amplifiers insystem10 could comprise a plurality of individual amplifiers working in conjunction, each amplifying a subset of the bandwidth processed by the single wide band amplifier. Alternatively, all amplifiers insystem10 could comprises wide bandwidth amplifiers. Depending on the overall bandwidth communicated bysystem10, one or more amplifier locations insystem10 could comprise a plurality of wide band amplifiers operating in conjunction to handle a total bandwidth significantly in excess of 100 nanometers. In other cases, a single wide band amplifier could process all traffic at a given location insystem10.

Wide band amplifiers withinsystem10 comprise multi-stage Raman amplifiers having at least two stages with approximately complimentary gain profiles. A combination of the complimentary gain profiles, in cooperation with any other gain stages in the wide band amplifier, results in approximately flat gain profile for the amplifier.

Throughout this description, the phrase “approximately complementary” refers to a situation where, at least in general,signal wavelengths116 that are highly amplified in the first stage are less amplified in the second stage, andsignal wavelengths116 that are highly amplified in the second stage are less amplified in the first stage. Two gain profiles said to be “approximately complementary” need not have equal and opposite slopes. Moreover, equal amplification of any particular wavelengths in both gain profiles does not preclude those gain profiles from being “approximately complementary.”

Conventional designs of multi-stage Raman amplifiers have been unable to process bandwidths in excess of 80 nanometers while maintaining approximately flat gain profiles and acceptable noise figures. One aspect of this invention recognizes that a major culprit in noise figures associated with conventional multi-stage Raman amplifiers is the phonon-stimulated optical noise created when wavelength signals being amplified reside spectrally close to pump wavelengths used for amplification. One aspect of the invention reduces adverse effect of this noise by enhancing the Raman amplification of signal wavelengths near the pump wavelengths to overcome the effects of the noise, and applying an approximately complementary Raman gain profile in another stage to result in an approximately flat overall gain profile.

FIG. 2 graphically illustrates the phonon-stimulated optical noise figure increase as the spectral spacing between signal wavelengths and pump wavelengths decreases. As shown in FIG. 2, phonon-stimulated noise increases dramatically as signal wavelength get close to the pump wavelengths.

One aspect of the invention significantly reduces adverse effects associated with phonon-stimulated noise by providing multiple stages of Raman gain having approximately complimentary gain profiles acting on substantially the same bandwidth of signals. While best results are obtained by applying approximately complimentary gain profiles to all or nearly all of the same signal wavelengths, some portion of wavelengths can be omitted from one gain profile and included in the other gain profile without departing from the scope of this invention.

FIG. 3ais a block diagram of an exemplary embodiment of a multiple stage Raman amplifier110 including gain profiles30 and40 associated with various amplification stages and anoverall gain profile50 for the amplifier. In this example,amplifier100 comprises a two-stage amplifier having afirst stage112 and asecond stage114 cascaded withfirst stage112. As will be further discussed below, the invention is not limited to a particular number of amplifier stages. For example, additional amplification stages could be cascaded ontosecond stage114. Moreover, although the illustrated embodiment showssecond stage114 cascaded directly tofirst stage112, additional amplification stages could reside betweenfirst stage112 andsecond stage114 without departing from the scope of the invention.

Amplifier100 could comprise a distributed Raman amplifier, a discrete Raman amplifier, or a hybrid Raman amplifier which comprises both discrete and distributed stages. Eachstage112,114 ofamplifier100 includes an input operable to receive a multiple wavelengthoptical input signal116. As a particular example,optical input signal116 could include wavelengths ranging over one hundred nanometers.

Eachstage112,114 also includes distributedgain media120,121. Depending on the type of amplifier being implemented,media120,121 may comprise, for example a transmission fiber, or a gain fiber such as a spooled gain fiber. In a particular embodiment,media120,121 may comprise a dispersion compensating fiber.

Eachstage112,114 further includes one or more wavelength pumps122. Pumps122 generate pump light124 at specified wavelengths, which are pumped into distributedgain media120,121. Raman gain results from the interaction of intense light from the pumps with optical phonons in silica fibers. The Raman effect leads to a transfer of energy from one optical beam (the pump) to another optical beam (the signal). Pumps122 may comprise, for example, one or more laser diodes. Although the illustrated embodiment shows the use of counter propagating pumps, under some circumstances using a relatively quiet pump, co-propagating pumps could also be used without departing from the scope of the invention.

In one particular embodiment, pump wavelengths124 can be selected so that the longest wavelength pump signal124 has a wavelength that is shorter than the shortest wavelength ofsignal116. As one specific example, the longest wavelength of pump light124 could be selected to be, for example, at least ten nanometers shorter than the shortest wavelength ofsignal116. In this manner,amplifier100 can help to avoid phonon stimulated noise that otherwise occurs when pump wavelengths interact with wavelengths of the amplified signal.

Couplers118band118ccouple pump wavelengths124aand124bto gain distributedmedia120 and121, respectively. Couplers118 could comprise, for example, wave division multiplexers (WDM) or optical couplers. A lossy element126 can optionally reside between amplifier stages112 and114. Lossy element126 could comprise, for example, an isolator, an optical add/drop multiplexer, or a gain equalizer.

The number of pump wavelengths124, their launch powers, their spectral and spatial positions with respect to other pump wavelengths and other wavelength signals, and the bandwidth and power level of the signal being amplified can all contribute to the shape of the gain profile for the respective amplifier stage. FIG. 3bshows exemplary gain profiles forfirst stage112 andsecond stage114.Gain profile30 shows the overall gain offirst stage112 ofamplifier100 for a bandwidth ranging from the shortest wavelength of signal116 (λ_sh) to the longest wavelength of signal116 (λ_lg).Gain profile40 shows the overall gain ofsecond stage112 ofamplifier100 for a bandwidth ranging from the shortest wavelength of signal116 (λ_sh) to the longest wavelength of signal116 (λ_lg). Each of gain profiles30 and40 reflects the effects of the other gain profile acting upon it.

In this example, gainprofile30 offirst stage112 has a downward slope, where a majority of theshorter signal wavelengths116 are amplified more than a majority of thelonger signal wavelengths116. Conversely, gainprofile40 ofsecond stage114 is approximately complimentary to gainprofile30 offirst stage112.Gain profile40 exhibits an upward slope where a majority of thelonger signal wavelengths116 are amplified more than a majority of theshorter signal wavelengths116.

Gain profile50 (shown in dotted lines in FIG. 3c) represents an exemplary composite gain profile ofamplifier100 resulting from the application of gain profiles30 and40 tooptical signal116.Gain profile50 is approximately flat over at least substantially all of the bandwidth of wavelengths withinsignal116.

In operation,amplifier100 receivesoptical input signal116 at distributedgain medium120 offirst stage112. Distributedgain medium120 could comprise, for example, a dispersion compensating Raman gain fiber, a transmission fiber, a high non-linearly fiber, a segment of transmission fiber, or combination thereof. Pumps122(a) generate pump wavelengths124(a) and apply them to distributed gain medium120 through coupler118(b). Pump wavelengths124 interact withsignal wavelengths116, transferring energy from the pump wavelengths124 to thesignal wavelengths116. In this example,shorter signal wavelengths116 are amplified more thanlonger signal wavelengths116 infirst stage112.

Amplified wavelengths ofsignal116 are communicated to distributedgain medium121 ofsecond stage114. Wavelengths ofsignal116 are amplified insecond stage114 by interacting withpump wavelengths124bgenerated atpumps122b. In this example, pumpwavelengths124boperate to result ingain profile40 where longer wavelengths ofsignal116 are amplified more than shorter wavelengths ofsignal116.

The combined effect of amplification infirst stage112 andsecond stage114 ofamplifier100 results in approximatelyflat gain profile50 across wavelengths ofoptical signal116. This particular example provides a significant advantage in reducing the noise figure associated with the amplifier. Using this configuration, the small signal noise figure ofamplifier100 can be reduced to less than eight decibels, in some cases7 decibels, even where the bandwidth ofsignal16 exceeds 100 nanometers.

FIG. 4ais a block diagram of another embodiment of a multiple stage Raman amplifier110 includingexemplary gain profiles130 and140 associated with various amplification stages and anoverall gain profile150 for the amplifier. Amplifier110 shown in FIG. 4 is similar in structure and function to amplifier100 shown in FIG.1. Likeamplifier100 shown in FIG. 1, amplifier110 of FIG. 4 includes a firstRaman amplification stage112 and a secondRaman amplification stage114. Each ofstages112 and114 includes a distributedgain medium120,121, respectively, which is operable to receive multiplewavelength input signal116 and pumpwavelengths124aand124b, respectively. Eachamplifier stage112 and114 operates to amplify wavelengths ofsignal116 according to gainprofiles130 and140 as shown.

The example shown in FIG. 4 differs from the example shown in FIG. 3 in that gain profile130 (shown in FIG. 4b) offirst stage112 exhibits an upward slope where a majority of longer wavelengths ofsignal116 are amplified more than the majority of shorter wavelengths ofsignal116. Conversely, gainprofile140 ofsecond stage114 comprises an approximately complementary gain profile tofirst gain profile130 offirst stage112. Inprofile140 applies a higher gain to a majority of shorter wavelengths than the gain applied to the majority oflonger signal wavelengths116. In addition, in this embodiment, the launch power ofpumps122adrivingfirst gain profile130 can be reduced.

This aspect of the invention recognizes that due to the Raman scattering effect, longer wavelength signals tend to rob energy from shorter wavelength signals. This aspect of the invention leverages that fact to allow the longer pump wavelengths ofwavelengths124ato rob energy from the shorter pump wavelengths ofwavelengths124b. In a particular embodiment, amplifier110 may include ashunt160 between second distributedgain medium121 and first distributed gain medium120 to facilitate the longer pump wavelengths ofwavelengths124aaccepting power from the shorter pump wavelengths ofwavelengths124b. The effects result in anoverall gain profile130 forfirst stage112 that remains approximately complimentary to the gain profile ofsecond stage140. As a result, the composite gain profile150 (FIG. 4c) of the amplifier remains approximately flat.

This embodiment provides significant advantages in terms of efficiency by allowing the use of fewer wavelength pumps122ain thefirst stage112, and/or also by allowing each pump122ato operate at a lower launch power.

The embodiment shown in FIG. 4acan also provide improvements for the noise figure of the amplifier. For example, phonon stimulated noise is created in Raman amplifiers where wavelengths being amplified spectrally reside close to a wavelength of pump signals124. One aspect of this invention recognizes that by spectrally separating pump wavelengths124 fromsignal wavelengths116, phonon stimulated noise can be reduced.

In a particular embodiment, pump wavelengths124 are selected to have wavelengths at least 10 nanometers shorter than the shortest wavelength inoptical signal116 being amplified. Moreover, in a particular embodiment,second stage114 where a majority of the gain to short wavelength ofsignal116 is applied comprises the last stage of amplifier110.

FIG. 5ais a block diagram of a threestage Raman amplifier200 includinggain profiles230,240, and245 associated with various amplification stages, and anoverall gain profile250 for the amplifier.Amplifier200 is similar in structure and function to amplifier100 of FIG. 3 but includes three cascaded amplification stages212,214, and215. Each of amplifier stages212-215 includes a distributedgain medium220,221,223, respectively, which operate to receive multiple wavelengthoptical signal216 and pump wavelengths224a-224cfrom pumps222a-222c. Each amplifier stage includes an optical coupler operable to introduce pump wavelengths224 to the respective gain media. In some embodiments, lossy elements226 may reside between one or more amplification stages212-215. Lossy elements226 may comprise, for example, optical add/drop multiplexers, isolators, and/or gain equalizers.

Amplifier200 may comprise a discrete Raman amplifier or a hybrid Raman amplifier. For example, first distributedgain medium220 may comprise a transmission fiber, a section of transmission fiber, or a Raman gain fiber. In a particular embodiment, first distributedgain medium220 could comprise a dispersion compensating Raman gain fiber.

Distributedgain medium221 ofsecond stage214 may comprise a segment of transmission fiber or a Raman gain fiber. Distributedgain medium223 of third amplifier phase215 could comprise, for example, a Raman gain fiber. In particular embodiments, any or all of distributed gain mediums220-223 could comprise a dispersion compensating Raman gain fiber.

In operation,amplifier200 receives signal216 atfirst stage212 and applies a gain to signalwavelengths216 according to gainprofile230 depicted in FIG. 5b.Signal216 next traversessecond stage214 wheregain profile240 is applied. Finally, signal216 is amplified by third stage215 according to gainprofile245 shown in FIG. 3b.Signal216 exitsamplifier200 atoutput260 having been exposed to acomposite gain profile250 as shown in FIG. 3c.

In this particular example,first stage212 andsecond stage214 operate in a similar manner to amplifier100 shown in FIG. 3a. In particular,first stage212 applies again profile230 that amplifies a majority ofshorter signal wavelengths216 more than it amplifies a majority oflonger signal wavelengths216.Second stage214, conversely, applies and approximatelycomplimentary gain profile240 to signal216, where the majority of longer wavelengths ofsignal216 are amplified more than a majority of shorter wavelengths ofsignal216.

The combination ofsecond stage214 and third stage215, on the other hand, operates similarly to amplifier110 shown in FIG.4. Whilesecond stage214 appliesgain profile240 amplifying a majority oflonger signal wavelengths216 more than a majority ofshorter signal wavelengths216, third stage215 applies to gainprofile245, which amplifies a majority ofshorter signal wavelengths216 more than a majority oflonger signal wavelengths216. In this particular example, gainprofile240 ofsecond stage214 is approximately complimentary to bothgain profile230 offirst stage212 and gainprofile245 of third stage215. In this example, the slope ofgain profile240 is significantly steeper than the slope ofgain profiles230 and245 to account for the fact thatgain profile240 is the only profile exhibiting an upward slope. The composite gain profile250 (shown in FIG. 5c) resulting from the combination of amplifications in first, second, and third amplifier stages ofamplifier200 results in an approximately flat gain profile.

This particular example reaps the efficiency benefits discussed with respect to FIG. 4, and permits use of the noise figure reduction techniques discussed with respect to FIGS. 3 and 4. For example, efficiency advantages are realized by allowing longer pump wavelengths224 ofsecond stage214 to accept power from high poweredshorter pump wavelengths224cof third amplification stage215. This results from the Raman effect wherein longer wavelength signals tend to rob energy from shorter wavelength signals. As a result,second stage214 can be operated with fewer wavelength pumps than what otherwise be required, and also with lower pump launch powers.

In terms of improvements in noise figure, the gain profiles offirst stage212 compared tosecond stage214 results in high amplification of shorter wavelengths ofsignal216 to overcome phonon stimulated noise associated with interaction of those signals with thelonger pump wavelengths224a. In addition, providing a significant amount of amplification to shorter wavelengths ofsignal216 in the last stage215 ofamplifier220 helps to minimize the noise figure associated withamplifier200.

FIGS. 6a-6cshow a block diagram of a four stage Raman amplifier, gain profiles associated with various stages of the amplifier, and a composite gain of the amplifier respectively.Amplifier300 is similar in structure and function toamplifiers100 and110 shown in FIGS. 1 and 2, respectively. In this example,amplifier300 includes four Raman amplification stages312,314,315, and317. Each amplification stage includes a distributedgain medium320,321,323, and325, respectively. Distributedgain medium320 offirst stage312 may comprise, for example, a transmission fiber or a Raman gain fiber. Each of distributed gain medium312-325 of second, third, and fourth stages314-317 may comprise a Raman gain fiber or a segment of transmission fiber. In particular embodiments, some or all of distributed gain media320-325 could comprise dispersion compensating Raman gain fibers.

Each distributed gain medium320-325 is operable to receive a multi wavelengthoptical signal316 and amplify that signal by facilitating interaction betweenoptical signal316 and pump wavelengths324a-324d. Pump wavelengths324 are generated by pumps322 and coupled to distributed gain media320-325 through couplers318. In this particular example, couplers318 comprise wave division multiplexers.

In the illustrated embodiment,amplifier300 includes at least one lossy element326 coupled between amplifier stages. In this example,lossy element326bcomprises an optical add/drop multiplexer coupled betweensecond stage314 andthird stage315. Optical add/drop multiplexer326bfacilitates mid-stage access toamplifier300 and allows selective addition and/or deletion of particular wavelengths fromsignal316. Other lossy elements, such as isolators or gain equalizers could alternatively reside between amplifier stages.

In operation, signal316 entersamplifier300 atcoupler318a, which passessignal316 tofirst amplifier stage312 where a gain profile at330, as shown in FIG. 4b, is applied to wavelengths ofsignal316.Signal316 is then passed tosecond stage314 where again profile335, as shown in FIG. 4bis applied to wavelengths ofsignal316.

In this particular example, first andsecond stages312 and314 ofamplifier300 operate similarly toamplifier100 described with respect to FIG.3. In particular,first stage312 applies a gain profile where a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths, andsecond stage314 applies an approximatelycomplimentary gain profile335 where a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths. In this particular embodiment, the composite gain fromfirst stage312 andsecond stage314 results in an approximately flat gain profile at the output ofsecond stage314. This design advantageously facilitates addition and subtraction of particular wavelengths ofsignal316 without the need for further manipulation of the gain. In addition, first and second gain stages312 and314 provide a low noise figure, reducing the effects of phonon stimulated noise in shorter wavelength signals closest to the pump wavelengths.

Continuing with the operational description, particular wavelengths ofsignal316 may be substituted with other wavelengths at add/drop multiplexer326b. After processing by add/drop multiplexer326b, signal316 continues tothird amplification stage315, wheregain profile340 is applied as shown in FIG. 6b.Signal316 is then communicated tofourth stage317 wheregain profile345 is applied to wavelengths ofsignal316. Amplifiedsignal316 is then output atoutput port365.

Third and fourth amplification stages ofamplifier300 are similar in structure and function to amplifier110 described with respect to FIG.4. Through the use of this configuration, third and fourth amplifier stages315 and317 provide increased efficiency in operation. In particular, pump322 can operate with fewer pump signals and/or lower launch power as a result of the Raman scattering effect which allowslonger pump wavelengths324cofthird stage316 to accept power from highly amplifiedshorter pump wavelengths324doffourth stage317. Moreover, third and fourth amplification stages315 and317 assist in maintaining a low noise figure by applying a significant amount of the gain to the shortest wavelengths ofsignal316 at thelast amplifier stage317.

FIG. 7 is a flow chart showing one example of amethod400 of amplifying a multi-wavelength optical signal using a multi-stage Raman amplifier. This particular example uses FIGS. 6a-6cto illustrate the method. Similar methods could apply to any of the embodiments described herein.Method400 begins atstep410 wherefirst amplifier stage312 receivessignal wavelengths316 and appliesfirst gain profile330 to those wavelengths. Step420 allows for optional mid-stage access betweenfirst stage312 andsecond stage314. The method continues wheresecond stage314 appliessecond gain profile325 to signalwavelengths316 atstep430.

Second gain profile335 is approximately complimentary tofirst gain profile330. In this particular example,first gain profile330 amplifies a majority ofshorter signal wavelengths316 more than a majority oflonger signal wavelengths316, whilesecond gain profile325 amplifies a majority of longer wavelength signals316 more than a majority of shorter wavelength signals316. Those gain profiles could be reversed if desired. Moreover, additional gain profiles could be applied betweenfirst stage312 andsecond stage314 by intervening stages (not explicitly shown). This particular example shows additional stages beyondfirst stage312 andsecond stage314. In a particular embodiment, an amplifier embodying the invention could comprise only two complimentary stages of Raman gain.

This example provides optional mid-stage access atstep450. Mid-stage access could comprise, for example, application of optical add/drop multiplexing, gain equalization, or the presence of one or more optical isolators.

Whereamplifier300 comprises more than two stages of complimentary Raman amplification,method400 continues atstep460 wherethird stage316 appliesgain profile340 to signalwavelengths316. Whereamplifier300 comprises a three stage amplifier,third gain profile340 can be complimentary tosecond gain profile335. An example of this operation is shown in FIG.5. Whereamplifier300 comprises a four stage amplifier,third stage315 can apply gain profile at340 as shown in FIG. 6b, whilefourth stage317 appliesgain profile345 as shown in FIG. 6batstep480.

In this example,third gain profile340 amplifies a majority oflonger signal wavelengths316 more than a majority ofshorter signal wavelengths316 whilefourth stage317 amplifies a majority ofshorter signal wavelengths316 more than a majority oflonger signal wavelengths316. In this manner, third and fourth stages ofamplifier300 can realize efficiency advantages by allowinglonger pump wavelengths324cfromthird stage315 to accept energy from highly amplifiedshorter pump wavelengths324dinfourth stage317.

Although this method has described a four stage amplification process, the method can equally apply to any system having two or more Raman amplification stages. In addition, although this particular example described first and second gain stages havinggain profiles330 and335 as shown in FIG. 6b, and third and fourth gain stages havinggain profiles340 and345 as shown in FIG. 6b, those gain profiles could be reversed without departing from the scope of the invention. The particular example shown provides significant advantages in a four stage amplifier in that initial stages can be configured to provide a low noise figure by emphasizing amplification of shorter wavelength signals early in the amplification process. In addition, third and fourth amplification stages advantageously realize efficiency gains in amplifier locations where noise reduction is not as critical a concern.

FIGS. 8a-8bare graphs showing simulations of one aspect of the present invention embodied in a two stage distributed Raman amplifier. FIGS. 9a-9bare graphs showing simulations of one aspect of the present invention embodied in a two stage discrete Raman amplifier. The parameters used for the amplifier simulations were as follows:

	Distributed	Discrete

Stage

1
Input Port Loss	0	dB	1.3	dB

Stage

1
Gain Fiber	80	km LEAF fiber	DK-21 (DCF)

Stage 1
Pump Powers:	438	mW @ 1396 nm
	438	mW @ 1416 nm	380	mW @ 416 nm
	438	mW @ 1427 nm	380	mW @ 1427 nm
	170	mW @ 1450nm	220	mW @ 1450nm
	10	mW @ 1472nm
	4	mW @ 1505 nm	19	mW @ 1505 nm
Mid-Stage Loss	2	dB	1.6	dB

Stage

2
Gain Fiber	DK-30 (DCF)	DK-19 (DCF)

Stage 2
Pump Powers:	380	mW @ 1399 nm
	380	mW @ 1472 nm	380	mW @ 1472 nm
	380	mW @ 1505 nm	380	mW @ 1505nm
Stage
2
Output Port Loss	1	dB	1.3	dB

FIGS. 8aand9A showfirst gain profile30 offirst stage112,second gain profile40 ofsecond stage114, andcomposite gain profile50 ofRaman amplifier100 for distributed and discrete configurations, respectively. As shown in these figures application of pump wavelengths124 as shown in Table 1 above results in a downwardly slopinggain profile30 forfirst stage112, and an upwardlysloping gain profile40 forsecond stage114. Gain profiles30 and40 are approximately complementary to one another, although they do not comprise mirror images of one another.

Thecomposite gain profile50 ofamplifier100 is approximately flat across the bandwidth ofsignal116 being amplified.Gain profile50 represents the gain profile without application of any gain flattening filters. In this embodiment,amplifier100 obtains an overall gain profile that is approximately flat for over 100 nanometers.

FIGS. 8band9bshow thesame gain profile50 and compare that profile to the noise figure of the amplifier. In the case of the discrete Raman amplifier simulated in FIG. 9b, the actual noise FIG. 55 is shown. In the case of the distributed Raman amplifier simulated in FIG. 8b, the effective noise FIG. 65 is shown.

An optical amplifier noise figure is defined as NF SNRin/SNRout where SNRin is the signal-to-noise ratio of the amplifier input signal and SNRout is the signal-to-noise ratio of the amplifier output signal. As defined, NF is always greater then 1 for any realizable amplifier. Effective noise figure for a distributed optical amplifier is defined as the noise figure a discrete amplifier placed at the end of the distributed amplifier transmission fiber would need to have to produce the same final SNR as the distributed amplifier. It can be, and in practice is, less than 1 (negative value in dB) for practical distributed amplifiers over at least a small portion of their operating wavelength range.

As shown in FIGS. 8band9b, the noise figure in this embodiment is always less than eight decibels over the entire bandwidth ofsignal116. In fact, for a bandwidth between 1520 nanometers and 1620 nanometers, the noise figure never exceeds 7 decibels.

Although the present invention has been described in several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the spirit and scope of the appended claims.

Claims (45)

What is claimed is:

1. A multi-stage optical amplifier, comprising:

a first Raman amplifier stage operable to amplify a plurality of signal wavelengths, the first Raman stage having a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and

a second Raman amplifier stage coupled to the first Raman amplifier stage and operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage, the second Raman stage having a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths;

wherein an overall gain profile for the amplifier over the at least a portion of the plurality of signal wavelengths is approximately flat.

2. The amplifier ofclaim 1, wherein the first and second Raman stages operate to amplify all of the same signal wavelengths.

3. The amplifier ofclaim 1, wherein one of the first and second Raman amplifier stages comprises a distributed Raman amplifier stage and wherein the other of the first and second Raman amplifier stages comprises a discrete Raman amplifier stage.

4. The amplifier ofclaim 1, wherein the overall gain profile of the amplifier is approximately flat over at least 40 nanometers of the plurality of signal wavelengths.

5. The amplifier ofclaim 1, wherein the overall gain profile of the amplifier is approximately flat over at least 60 nanometers of the plurality of signal wavelengths.

6. The amplifier ofclaim 1, wherein the overall gain profile of the amplifier is approximately flat over at least 70 nanometers of the plurality of signal wavelengths.

7. The amplifier ofclaim 1, wherein the overall gain profile of the amplifier is approximately flat over at least 80 nanometers of the plurality of signal wavelengths.

8. The amplifier ofclaim 1, wherein the overall gain profile of the amplifier without the use of a gain flattening filter would vary by less than five decibels over the at least 60 nanometers.

9. The amplifier ofclaim 1, wherein the overall gain profile of the amplifier without the use of a gain flattening filter would vary by less than three decibels over the at least 60 nanometers.

10. The amplifier ofclaim 1, wherein the overall gain profile of the amplifier without the use of a gain flattening filter would vary by less than one decibel over the at least 60 nanometers.

11. The amplifier ofclaim 1, wherein the gain profiles of the first and second Raman amplifier stages are each determined at least in part by at least some of a plurality of pump wavelength signals, and wherein the plurality of pump wavelength signals comprise a shortest pump wavelength and a longest pump wavelength.

12. The amplifier ofclaim 11, wherein a highest level of gain supplied by the longest pump wavelength is supplied in a last Raman amplifier stage of the amplifier.

13. The amplifier ofclaim 11, wherein the first Raman amplifier stage comprises an initial Raman stage of the amplifier and wherein the first Raman stage operates to apply a higher gain level to a signal wavelength closest to the longest pump wavelength than a gain applied to a signal furthest from the longest pump wavelength.

14. The amplifier ofclaim 11, wherein the longest pump wavelength that provides Raman gain to at least a portion of the signal wavelengths comprises a wavelength at least 5 and no more than 50 nanometers shorter than the shortest wavelength of the plurality of signal wavelengths.

15. The amplifier ofclaim 1, wherein each of the first and second Raman amplifier stages comprises a gain fiber and wherein a small signal noise figure developed in of the gain fibers of the first and second amplifier stages is no greater than 6 decibels over at least 60 nanometers of the plurality of signal wavelengths.

16. The amplifier ofclaim 1, wherein an increase in noise figure of the amplifier due to phonon stimulated noise comprises no more than four decibels.

17. The amplifier ofclaim 1, wherein the gain profiles of the first and second Raman amplifier stages comprise approximately complementary gain profiles.

18. The amplifier ofclaim 1, wherein at least two stages of the amplifier are coupled so as to cause longer pump wavelengths supplied to one of the stages to accept power from shorter pump wavelengths supplied to another of the stages.

19. The amplifier ofclaim 1, wherein at least one of the first Raman amplifier stage and the second Raman amplifier stage imparts a net gain to at least a portion of the plurality of signal wavelengths.

20. The amplifier ofclaim 1, further comprising at least one additional amplifier stage operable to amplify at least some of the plurality of signal wavelengths before those signal wavelengths are received by the first Raman amplifier stage.

21. The amplifier ofclaim 1, further comprising at least one additional amplification stage coupled between the first and second Raman amplification stages.

22. The amplifier ofclaim 1, further comprising a rare earth doped amplifier stage coupled to at least one of the first or second Raman amplifier stage.

23. The amplifier ofclaim 1, further comprising a gain flattening filter coupled to the amplifier, the gain flattening filter operable to further flatten the gain profile of the amplifier.

24. The amplifier ofclaim 1, further comprising a third Raman amplifier stage coupled to the second Raman amplifier stage and operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the second stage.

25. The amplifier ofclaim 24, wherein the third Raman amplifier stage comprises a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths.

26. The amplifier ofclaim 24, wherein the third Raman amplifier stage comprises a gain profile wherein a majority of the shorter signal wavelengths are amplified more than a majority of the longer signal wavelengths.

27. A method of amplifying an optical signal having multiple wavelengths, the method comprising:

amplifying a plurality of signal wavelengths at a first Raman amplifier stage having a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and

amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage after those signal wavelengths have been amplified by the first stage, the second Raman amplifier stage having a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths;

wherein an overall gain profile for the amplifier over the at least a portion of the plurality of signal wavelengths is approximately flat.

28. The method ofclaim 27, wherein the first and second Raman stages operate to amplify all of the same signal wavelengths.

29. The method ofclaim 27, wherein the overall gain profile of the amplifier is approximately flat over at least 40 nanometers of the plurality of signal wavelengths.

30. The method ofclaim 27, wherein the overall gain profile of the amplifier is approximately flat over at least 60 nanometers of the plurality of signal wavelengths.

31. The method ofclaim 27, wherein the overall gain profile of the amplifier is approximately flat over at least 70 nanometers of the plurality of signal wavelengths.

32. The method ofclaim 27, wherein the overall gain profile of the amplifier is approximately flat over at least 80 nanometers of the plurality of signal wavelengths.

33. The method ofclaim 27, wherein the overall gain profile of the amplifier without the use of a gain flattening filter would vary by less than five decibels over the at least 60 nanometers.

34. The method ofclaim 27, wherein the overall gain profile of the amplifier without the use of a gain flattening filter would vary by less than three decibels over the at least 60 nanometers.

35. The method ofclaim 27, wherein the overall gain profile of the amplifier without the use of a gain flattening filter would vary by less than one decibel over the at least 60 nanometers.

36. The method ofclaim 27, further comprising applying a gain flattening filter to at least some of the plurality of signal wavelengths to further flatten the overall gain profile of the amplifier.

37. The method ofclaim 27, further comprising, at the first Raman amplifier stage, applying a higher gain level to one of the plurality of signal wavelengths closest in wavelength to a longest pump wavelength than a gain applied to one of the plurality of signal wavelengths furthest in wavelength from the longest pump wavelength.

38. The method ofclaim 27, further comprising applying in a last Raman amplifier stage, a highest level of gain supplied by a longest pump wavelength signal that provides Raman gain to at least a portion of the plurality of signal wavelengths.

39. An optical amplification system, comprising:

a plurality of optical amplifiers coupled to an optical communication link, each operable to receive at least a majority of a plurality of wavelength signals, wherein the plurality of optical amplifiers comprises:

a first Raman amplifier comprising a gain profile wherein a majority of shorter wavelength signals are amplified more than a majority of longer wavelength signals; and

a second Raman amplifier comprising a gain profile wherein a majority of longer wavelength signals are amplified more than a majority of shorter wavelength signals;

wherein an overall gain profile for the plurality of wavelength signals after traversing the optical link is approximately flat over at least a portion of the plurality of wavelength signals.

40. The system ofclaim 39, wherein the overall gain profile for the plurality of wavelength signals after traversing the optical link is approximately flat over at least 40 nanometers of the wavelength signals.

41. The system ofclaim 39, wherein the overall gain profile for the plurality of wavelength signals after traversing the optical link is approximately flat over at least 60 nanometers of the wavelength signals.

42. The system ofclaim 39, wherein the overall gain profile for the plurality of wavelength signals after traversing the optical link is approximately flat over at least 80 nanometers of the wavelength signals.

43. The system ofclaim 39, wherein the overall gain profile over at least 60 nanometers of the plurality of wavelength signals after traversing the optical link would vary by less than five decibels without the use of a gain flattening filter.

44. The system ofclaim 39, wherein the overall gain profile over at least 60 nanometers of the plurality of wavelength signals after traversing the optical link would vary by less than three decibels without the use of a gain flattening filter.

45. The system ofclaim 39, wherein the overall gain profile over at least 60 nanometers of the plurality of wavelength signals after traversing the optical link would vary by less than one decibel without the use of a gain flattening filter.

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